The coupon box can be disabled from the Joomla administrator area->Components->VirtueMart->Configuration->Global.
There is a check box labelled Enable Coupon Usage. Using it you can enable and disable the coupon box, which is shown during the check out procedure. It can not be enabled just for a single page and hidden in the others.
You can also manage your coupons through the Joomla administrator area->Components->VirtueMart->Coupon.
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